This invention relates to a normally disengaged electromagnetic friction clutch of the type in which a rotatable driving member with a generally cylindrical hub is adapted to be coupled to a driven member when a shoe is contracted radially into frictional gripping engagement with the hub. The shoe is surrounded by a helically coiled spring having one end connected to the driven member and having an opposite or armature end which is adapted to be attracted axially into engagement with the rotatable driving member when an electromagnet is excited. As an incident thereto, pilot torque is transmitted magnetically between the driving member and the armature end of the spring to turn the armature end of the spring through a short distance. This causes the coils of the spring to wrap down around the shoe and contract the shoe into frictional gripping engagement with the hub of the driving member. The shoe thus rotates with the hub and transmits torque to the driven member.
The pilot torque which is transmitted magnetically between the driving member and the armature end of the spring is of relatively small magnitude and is transferred directly through the spring to the driven member. The pilot torque is amplified, however, by the action of the spring wrapping downwardly around and contracting the shoe and thus a high percentage of the total transmitted torque is transferred between the driving and driven members by way of the shoe.
Clutches of this general type are disclosed in Mason U.S. Pat. No. 3,149,706 and Baer U.S. Pat. No. 3,685,622. In the Mason clutch, the shoe is rotatable relative to the driven member and the helical spring and causes the spring to self-energize. In other words, the spring frictionally engages the shoe and wraps down even more tightly than the wrap which is effected by the magnetic pilot torque between the driving member and the armature end of the spring. As a result, the torque which is transmitted directly through the spring is greater than the pilot torque and is not controlled solely by the pilot torque.
Self-energization of the helical spring also occurs in the Baer clutch. In that clutch, the spring wraps down around the driving hub as well as around the shoe. Frictional engagement of the spring with the driving hub causes the spring to wrap more tightly than the wrap which is produced by the magnetic pilot torque and thus the torque which is transmitted directly through the spring is greater than and is not directly related to the pilot torque.
In a conventional helical spring clutch in which the spring spans the gap between the driving and driven members, the first coil of the spring wraps downwardly on and engages the driving member with high friction and increases the torque in the remaining coils which surround the driving member. Accordingly, the next coil wraps downwardly even tighter on the driving member and increases the torque in the remaining coils still further. The last coil which surrounds the driving member is subjected to the highest torque. Thus, the maximum torque which can be transmitted by the clutch is limited by the torque which can be carried by the last coil on the driving member.